RTS -qa option is helpful but under-documented
The entire documentation for the RTS
-qa option is
Use the OS's affinity facilities to try to pin OS threads to CPU cores. This is an experimental feature, and may or may not be useful. Please let us know whether it helps for you!
It does help for us: enabling it on a multithreaded, moderately GC-heavy web server led to a sizeable improvement (~20%) in throughput benchmarks. However, it would be nice if the GHC documentation was a little more descriptive about what exactly this option does and how experimental it is now considered! (Relatedly, perhaps it would be worth adding a single index of RTS options in the user's guide, much like the flag reference for GHC itself.)